Signaling system



Nov. 9, 1943. 1R. A. Fox

' SIGNALING SYSTEM Filed oct. 17. 1940 4 Sheets-Sheet 1 Nov. 9, 1943. R. A. Fox

SIGNALING SYSTEM Filed Oct. 17, 1940 4 Sheets-Sheet 2 RBERTA. FOX

Nov. 9, 1943. R. A. Fox 2,333,992

SIGNALING sysmn Filed oct. 17, 1940 4 sheets-sheet s M 11M/ms s :www

Nov. 9, 1943. R. A. Fox 2,333,992

SIGALING SYSTEM Filed Oct. 17. 1940 4 Sheets-Sheet 4 /15 N6 R. f: DETECTOR ,wn/0 Dtrfcmk y" l?. AMPLIFIER l AMJ call VERTER AMP/ mm Q z s 24a om 45 Y zu musi' 244 ('A 247 4l f'Af) DIAL S TIPP/IVG 1 264 [NYM/70K )70551974 FX Patented Nov. 9, 1943 SIGNALING SYSTEM Robert A. Fox, Cleveland Heights, Ohio, assigner to Herman E. Hageman, trustee, Lorain, Ohio Application October 17, 1940, Serial No. 361,601

(Cl. Z50-6) Claims.

In the art of signaling, particularly in remote control systems, there has arisen a desirability or necessity for transmitting and receiving impulses over telephonie carrier' channels. When these impulses are superposed on carrier frequencies interference may be caused between the telephonie and impulse carriers, or their modulations, to such an extent as to distort voice communication or cause a false operation of the remotely positioned relays or switches. Whereas in the prior art discrimination or selectivity between the carriers is obtained on the basis of differences of amplitude of the transmitted or received currents or voltages, in accordance with the present invention an entirely different method and apparatus are employed to effect the necessary selectivity between the carriers and their side bands. L

In brief, my invention contemplates the use of frequency discrimination as contrasted with selectivity obtained solely by differences of current or voltage amplitudes. In carrying out my invention I provide a source of frequencies which consists of a base frequency and an incremental frequency either added to or subtracted from the base frequency, which when acted upon by a frequency sensitive detector will be converted into direct currents of opposite polarities .to which the final receiving devices respond selectively.

Accordingly, the primary object of the present invention is to provide an improved signaling and remote control system for use either in radio or wired circuits.

Another object is to provide a circuit of the character referred to, which is sensitive to changes in the frequency but not in the amplitude of the transmitted or received impulses.

Other objects are to provide an improved system for dialing or operating teletype machines over a radio carrier wave without interference with the voice modulation or other tone modulation being transmitted simultaneously; to provide a composite telephone and telegraph or other impulse system of the carrier wave type, in which static is prevented from affecting the telegraphic impulses being transmitted through the system. Other objects and features'will be apparent as the specification is perused in connection with the accompanying drawings.

In the drawings:

Figure 1 is a circuit illustrating a frequencydetecting device which is used to advantage in effecting complete discrimination between the carrier waves of a composite transmitting or receiving system.

Figure 2 shows a modified form of the circuit shown in Figure 1.

Figure 3 illustrates a superheterodyne radio receiver which employs a frequency sensitive detector to provide frequency discrimination.

Figure 4 is a circuit somewhat similar to Figure 3, but employing two antennae and interconnections between the two receiving circuits.

Figure 5 shows the application of the invention to a wired link, including a superheterodyne receiver, and connected between a receiving antenna and a transmitting antenna.

Figures 6, 7 and 8 illustrate the manner in which the improved frequency discriminating circuit, and` including the frequency sensitive detector, may be employed to energize and actuate relays; selectors or other mechanism.r

Figure 9 shows a circuit employing the frequency sensitive detector and so arranged as to prevent fading or to prevent changes in signal amplitude from having any effect on the output voltage.

Figure 10 is similar to Figure 6 except that an additional lter is added to the frequency sensitive detector.

Figure 11 shows a transmitting circuit including 'a dialswitch for transmitting selective impulses over a radio link to a receiving station, which has a contact bank and responds to the impulses transmitted by the dial. The receiving circuit is provided with the improved frequency detector and is shown in Figure 14.

Figure 12 shows a switching mechanism at a transmitter station for controlling the rotation v of incremental and decremental frequencies which affect the receiving circuit as impulses of different polarities.

Figure 13 illustrates the use of a crystal for controlling the frequencies radiated by the system shown in Figure 12.

Figure 14 shows a superheterodyne receiving circuit provided with the improved frequency detector for actuating receiving dial mechanism, or a contact bank in response to transmitted impulses originating at a circuit similar to that shown in Figure 11.

In telephone practice over wired conductors, and in other electrical fields, relays, switches, selectors and similar apparatus are operated by electric currents. Direct current or alternating current of high and low frequency, also direct current of a polarity reversing character maybe employedfor this purpose. fHowevenin applying these methods to a radio carrier wave, direct current cannot be used but instead only alternating currents can be superposed on the carrier wave. The carrier wave itself may be transmitted in impulses, and these rectified at the receiver into direct current impulses, but the polarity cannot be reversed, and under such con-V ditions the carrier wave cannot be used for transmitting voice modulations at the same time.

Alternating current may vbe applied as modulations superimposed on a radio carrier wave, and several such alternating currents of different frequencies may be transmitted simultaneously and separated at the receiver and rectied into direct currents which may have different polarities. However, this interferes with voice j transmission which it may be desirable to transmit at the same time, unless very elaborate wave bands or elimination filters are employed. The device which will be described presently makes it possible to transmit the equivalent of direct current impulses or currents of a continuous nature without the use yof alternating current modulation. Furthermore, thev polarity of the direct current may be changed, and this may also be accomplished at Vthe same time that voice transmission is taking place, without 'any interference with the voice transmission or other modulation. The eiilcacy of the improved system to be described presently depends on the use of a fre'- quency sensitive detector, such for example, as is shown in'Figures l and 2. v

In Figure 1 the numeral I `designates the envelope'of anordinary tetrode amplifying tube containing an indirectly heated cathode' 2, a

heater 3,'a control grid l, a screen grid 5 and a plate 8. The cathode 2 is connected through a 'resistor 1 to ground 8, the resistor being shunted by a condenser 8. There is a resistor I8 shunted between the control" grid conductor 4II and the Acathode conductor I2. A condenser I3 is connected between the upper end of the resistor I0 and the input terminal I4. The other input terminal is indicated at I5. I y

'I'he output circuit of theY tube I contains a pair o1'y transformers I6, I1 which are tuned to diierent frequencies by the condensers I8, the upper ends of the primaries of these transformers being connected to the plate i by a conductor I9, and the lower ends of the primaries being connected to the cathode 2 through a resistor 20 and a condenser 2|. The plate potential is derived from a battery 22, poled as indicated, and

connected between the lower ends of the primaries of the transformers I6, I1 at a point between the condenser 2| and the resistor 1. In addition to the amplier tube I, the circuit employs a double unit rectii'lerindicated at 23, of

l any suitable and well known type, and ,containing a pair of plates 24, 25 respectively, which cooperate with a pair of indirectly heated cathodes 26, 21. Heatersy 28 may be providedfor the cathodes.

hereinafter. Resistors 34 are connectedin shunt windings of the transformers I6, I1.

to the condensers 3|, these resistors being .connected at the middle to form a terminal 35' A circuit, such as described, has kthe ability to receve changes in frequency and obtain from such changes direct current voltages of different po- .Y

Any alternating current of either high larities. or low frequency may be connected to the input terminals I4, I5 and thevoltage is impressed on the grid circuit of the amplifier tube- I, this grid being biased by the condenser 8 and the cathode resistor 1.

`The pulsating current which appears in the plate circuit 6 is fed into the two tuned parallel n 'I'hese Wind-- ings are adjustably coupled, as indicated by the arrows, and each of them is tuned by its respective condenser I8 to a frequency which is diiferent from the original frequency. For example, the secondary of the transformer I6 may be tuned by its condenser I8 to a frequency 1000 cycles higher than the frequency to which the primary winding is timed by its condenser I8, and the secondary of the transformer I1 may be tuned by its'condenser Ilto a frequency 1000 cycles lower than that to which its primary winding is tuned. The tube 23 is employed as a rectier, the condensers 3| serving as alternating currentk by-passes, while the resistors 34'V serve, in effect, `as potentiometer: for equally dividing the potential applied to the plates 24.

The voltages across the secondaries of the transformersv I8, I1; will be the same for the Irequency to which the primary windings are tuned, so that whenrectied by the tube 23 and fed in opposite directions through the resistors 34,`they will oppose each other and cancel out. Under .33 since cancellation does not take place at rthe resistors 34. If the frequencyacross the primary were then changed to a value lower by 1000 cycles ,i than the original base value, then the secondary of the transformer I 81s out of tune and the secondary winding of the transformer I1 is intime, so thatcurrents are again unequal and a voltage appears across the terminals 33, 'but of opposite polarity than before.` Thus by Vshifting .the frequency from a certain center or base frequency to one higher, and then to one'lower, impulses of kdirect current voltagewill appear across the terminals 33, and these kimpulses can be used to operate other equipment vfor remote control or signaling purposes. f

Figure 2 is a syste'mwhich operates similarly to Figure 1, except that it employs only a single transformer instead of a pair of transformers. This transformer is indicated at 38, 'the primary of which is tuned by a variable condenser 31, and the secondary by a condenser 38. The remaining parts, which are similarto parts of Figure 1, have been given corresponding reference numerals. These elements -need yno further explanation.

There is a doublel pole rotary condenser 38 con` nected across the variable condenser 38 and grounded as indicated at 40. A wire 4I is connected between the middle terminal 35 and a conductor 42 which is connected to the mid tap on the secondary of the transformer 38 for the purpose of supplying a direct (current path for the rectified current back to the plates. A condenser 43 is connected between the conductor 42 and the plate 6 of the tube I in order to isolate the lastmentioned plate potential from the secondary of the transformer 3B.

It is apparent that by rotating the movable element of the condenser 38, the secondary of the transformer 36 can be first tuned to a frequency greater than a given or base frequency and then to a frequency less than the base frequency with exactly the same effect as in Figure 1, in which impulses of direct current voltage will appear across the terminals 34 and having opposite polarities. A suitable transmitting circuit for generating the various frequencies necessary for the operation of the circuits shown in Figures 1 and 2 Will be described in connection with Figure 12. Condenser 39 serves to balance the circuit to ground so as to have only inductive coupling be.- tween the primary and secondary of transformer 36.

The rectangles 44, 45 and 45 indicate in a general way three crystal controlled, or other type of master frequency controlled oscillators of any suitable and well known type, in which the rectangle 45 indicates the oscillator of the base frequency, the rectangle 44 the oscillator of the plus incremental frequency and the rectangle 48 of the minus incremental frequency. The output of each of the oscillators is connected throughV any suitable form of amplifier 41 to the contacts 48, or any standard form of multiple switch. The output conductor of the switch is indicated at 49, and passes into a multiple-stage power amplier indicated at 50. The output of the amplier is connected in any suitable and well known manner to an antenna 5| and a ground indicated at 52. The rectangle 53 designates a modulating tube, the output of which is fed through the conductors 54 into the power ampliiler 50. Wires 55 pass to any suitable circuit on which the electrica] impulses corresponding to Voice undulations can be impressed.

Assume that the difference frequency between the oscillators 44 and 45 is 1000 and the difference frequency between the oscillators 45, 45 is 1000-, the oscillator 45 in both cases being considered the base frequency from which these differences are measured, the three frequencies are amplified by their respective amplifiers 41 and applied to the contacts of the switch 48, Thus by moving the switch in any suitable manner, the 1000 cycle plus frequency is first passed through the power amplifier 50, radiated at 5I, and then the 1000 cycle minus frequency is given the same treatment. Now let us assume that the circuit shown in Figure 1 is connected to a receiving antenna and hence is adapted to receive the two frequencies generated at the rectangles 44 and 46, for example, through a dernodulating oscillator and a radio frequency detector. Certain forms of a suitable receiving circuit will be explained presently in connection with Figures 3 and 4.

The switch 48 (Figure 12) may be moved in any suitable manner, for example, by a continuously operating contact making and breaking device, or by a relay operated from a telephone dial or tape, or by manual operation. A systematic code may then be transmitted, consisting of three different frequencies, namely H-Af, f and f-Af, transmitted in various combinations. If the frequency f-l-Af is alternated with the frequency f-Af, there will appear across the terminals 33, in Figures 1 and 2, direct current potentials which reverse in polarity at the same rate as the reversals which take place at the switch 48. The secondary of 36 is always tunedto the base frequency and changing to f-l-Af and f-Af causes the secondary to be out of tune for the f-l-Aj and f-ay conditions. This detuning causes the and voltages to appear when f-l-Af and ,f-A/ are transmitted. Itis apparent that the reversals of the direct current potentials at the terminals 33 give satisfactory selection methods of operating two relays, one of which responds to potentials of one polarity and the other responding to potentials of the other polarity. Such a system will be explained in connection with Figures 8 to 10, inclusive, and Figure 14. While the usual method of operating the transmitter would be to alternate the incremental frequency as applied to the antenna with the decremental frequency, it will be understood that if desiredthe incremental frequency may be transmitted as Ysucceeding impulses.

If desired, crystal controlled oscillators may be employed at the rectangles 44, 45 and 46, the crystals of each oscillator being shunted by an adjustable condenser the variations of which control the frequencies generated by the respective oscillators. Such a system is shown in Figure 13, in which the variable condensers are indicated at 56, 5l and 58. A master oscillator is controlled as to frequency by the crystal 58, and changes in the transmitted frequency may be brought about by the variable condensers 55, 51 and 58 which are shunted to ground in any suitable manner across the oscillator and controlled by switches, as is well known in the art. It will be understood that there is provided at a suitable point a switch similar to the element 48 in Figure 12, which serves to connect the output of the master oscillator to the antenna 60 through a suitable power amplifier apparatus 6 l, and simultaneously controlling the frequency at the condensers 55, 5l, 58 which is delivered to the power amplifier oscillator apparatus. The latter may also receive voice current modulations from the devices contained in the rectangle 53.

In Figures 3 and 4 typical forms of receiving circuits are illustrated, including the frequency sensitive detector shown in Figures 1 and 2 to obtain selective operation of direct current relays at a remote position, and controlled by one of two frequencies originating at a transmitter station, In Figure 3 the receiving antenna is indicated at 62, and is connected to a superheterodyne receiving circuit including a radio frequency detector and demodulator, contained in a rectangle The output of the detector feeds an audio frequency detector indicated by the rectangle 64, and is connected to a reproducer indicated by the rectangle B5 of any suitable type. From a position between the rectangles 63 and 64 there is taken a pair of conductors which feed the input circuit of a radio frequency detector indicated by the rectangle 65, such for example, as is illustrated in Figure l. The output of the frequency sensitive detector may be connected to a switching mechanism indicated by the rectangle 61 of a type which will be described presently, this switching mechanism being so arranged as to perform a desired function on the reproducer, as indicated by the dot-dash lines 68.

Assume that the antenna 62 receives in succession the three frequencies generated in the rectangles 44, 45 and 45 (Figure l2). The voice modulated carrier which is the frequency modulated carrier passes through the radio frequency audio frequency detector 84, into the reproducer 65. However, the impulse modulated intermediate frequency is taken from the main receiving circuit at the conductors 69 and fed through the frequency sensitive detector 66, and finally toA a switching mechanism 61.,

It is apparent that the detector and demodulator 63 serves merely to lower the frequency spectrum of the frequencies f-l-Af, f or f-Af, but all of the original modulation side bands are retained andV any frequency changes of a small amount, such as Af, are changed into correspondingv changes at the new frequency, the only difference being the percentage difference between the frequency change and the new carrier frequency produced by heterodyning. For example, if the frequency of the impulse transmitter was originally 100 cycles from a center or base frequency of 3,000,000 cycles, and this 3,000,000-cycle wave is converted to an intermediate frequency of 450,000 cycles at the receiver, then the frequency change will still be 1000 cycles above and below the new carrier frequency of 450,000 cycles. The

frequency shift of 1000 cycles is not divided in the same manner as the original carrier frequency is divided, but remains the same.

It was pointed out in Aconnection with Figure 1 that when f-Aj frequency is applied to the frequency sensitive detector, a direct current potential will appear'at the upper terminal 3| of the outgoing'line. f+Af is applied to the detector a direct current potential of negative polarity will appear at the upper terminal 3|. The base frequency f, upon reaching the frequency sensitive detector, will be balanced out at the resistors 34 and therefore no voltage will `reach any switching mechanism which is connected to the frequency sensitive detector.` The switching mechanism 61 may include a pair of relays, one of which responds to direct current potentials of one polarity and the other vresponding to potentials of the opposite polarity. It is apparent that these vrelays may be so connected with the reproducer, which in the Ycase of .Figure 3 might be a loud speaker, as to effect any kind of' control over the loud speaker. Such a control might be the closing of the circuit of the loud speaker or a change in volume delivered by the loud speaker, over which absolute control is exercised by the frequencies f-l-Af and f-Af generated atthe transmitting station and transmitted in any desired sequence.

In Figure 4 there is shown a receiving circuit in which the signaling and audio receiving channels are separated, and each passes through its individual radio frequency detector and demodulator apparatus in order to perform functions which might not be accomplished by the system shown in Figure 3. It is apparent that the same considerations of reducing the carrier frequency by the heterodyning effect apply to the circuit shown in Figure 4 as in the case of the circuit shown in Figure 3. The dot-dash lines 68 indi- .cate the same function as in Figure 3, namely, that connections may be taken from the switching mechanism 61 to perform any desired function with respect to the reproducer 65. l

The heterodyning principle has been carried one step farther in Figure 5, in that the use of the frequency sensitive detector B5 has been shown in connection with a wire linkl between the On the .other hand, when a frequency receiving antenna 62v and a transmitting antenna 18. As in the case of Figure 3,.the frequency sensitive detector 68 and the audio frequency detectcr 64 are connected to the common heterodyning apparatus 63, and the output of the reproducer 65 is taken to a suitable and well known form of radio transmitter 1| including an oscillator. The switching mechanism 61 is connected to the output circuit of the frequency sensitive detector, and may be arranged to control certain functions in the transmitter 1| through the wires 12. The dot-dash lines 88 indicate jthatthe switching mechanism 61 may also serve some useful control functionfwith respect to the reproducer 65 and also the transmitter 1|. It-is apparent that in the system shown in Figure 5 complete vcontrol of the transmitter 1| may be exercised from the transmitting end of a radio link, of which the antenna 62 is the receiver.

` In Figures 6, '7 and 8 I have shown various forms of switching mechanismr which lend themselves to operation when energized by positive and negative polarity impulses appearing at the upper terminals of the frequency sensitive detectors shown in vFigures 1 and 2. The detector is indicated at 13, and it willlbe understood that the detector is preceded ,by suitable heterodyning apparatus in case the ,f-i-Aj and j-Af frequencies are transmitted as carrier frequencies. Across the cathodes 26, 21 there is connected a pair of resistors 1,4 which are shunted by condensers 15. A wire 16 is taken to the mid tap of the secondary of a transformer 36 (Figure 2)y or to any suitable mid-point in the plate circuit of the frequency sensitive detector of Figure 1. `The wire 16 is extended into a wire 11 from which the voice modulation may be obtained. The output of the frequency sensitive detector in Figure 6 is fed into a. pair of electromagnets 18, 19 provided with coils and 8|, respectively.v A conductor 82 is connected between the upper ends ofthe coils, 8| and the cathode 26 of the detector, while ythe conductor 83 is connected between the lower ends of said coils andthe other cathode 26'of the detector. The armature of the electromagnet 18 is extended to receive a coil 84 which is wound in the same direction as the coil 80, and forms part of a circuit including a battery 85. The latter is so poled as to make the upper portion of the coil 84 negative. 'I'he core of the electromagnet 19 is similarly extended to receive a coil 86 Wound in the same direction as the coil 8|, the coil 86 being connected to the battery 85 in such a way as to make the upper portion` of the coil positiverwith respect to the lower portion thereof. There is 'a springarrnature 81 directly below the electromagnet 18 and responsive to the electromagnetic eld, this armature being adapted to swing between a pair of stationary contacts 88 and 89 making alternate contact therewith. There is a corresponding spring armature 98 positioned directly below the electromagnet 18, and this armature is 'adapted to swing between a pair of stationary contacts 9|, 92. It willbe understood that the armatures 81, 90, also the contacts 88, 89, 9| and-92 are connected to external circuits which may include relays. Assuming that there is an amplifier preceding the frequency sensitive detector 13, as in the case of Figure l, this output current is rectified in the detector 13 and passed through the resistors 14 and filtered by the condensers 15.

"I'husl there will appear at the point la an output voltage which is positive for one frequency change and negative for the other frequency change, and zero for the center frequency (assuming that the three frequencies f-l-Af, f and f-Af have been received by the amplier, using a heterodyning effect in case these frequencies are superposed on a carrier. At the point b there will appear a direct current voltage and an audio frequency voltage if the carrier were originally modulated. Only the voltage at a is employed for the operation of the electromagnets 18 and 19. The voltage from the battery 85 is applied to the electromagnets in such a way as to polarize them in different directions so that they will differentiate between positive and negative potentials. It is apparent that the current from the battery passes through the coils 84, 80 in opposite directions with respect to their electromagnets.

Consequently, in order to operate the electromagnet 18 it is necessary that the current passing through the coil 80 shall be in the same direction as the current passing through the coil 84, and this current must be in the opposite direction to the current which passes through the coil 8| for energizing the electromagnet 19. The electromagnet 10 will therefore pull up the armature 81 when the potential at the point a is such as to make the upper portion of the coil 80 negative. The electromagnet 1S will pull up the armature 90 when the potential at the point a is such as to make the upper portion of the coil 8| positive. The make and break operation which takes place at the armatures 81, 90 may be used to perform many operations under the control of a distant transmitter.

Still another way in which the frequency sensitive detector 13 (Figure 6) can be arranged selectively to operate two relays is shown in Figure 7. The output of the detector 13 is caused to be fed into a pair of thermionic tubes or relays 93, 94 provided with cathodes 95, 96, control grids 91, 98 and plates 99, |00. A resistor and a battery |02 are connected in series across the gridcathode circuit of the tube 93, and a resistor |03 is connected directly across the grid-cathode circuit of the tube 94. The battery |02 is so poled as to make the grid 91 negative with respect to its cathode, and a connection is made from the negative side of the battery through a conductor |04 to one of the -cathodes of the detector 13. The grids 91, 98 are connected together, andalso connected through a conductor |05 to the point a which leads to the other cathode of the detector. The output circuits of the tubes 93, 94 include a plate battery |06 and an electromagnet |01. Directly below these electromagnets there is a pair of spring armatures |08 adapted to swing between a pair of fixed contacts |09.

As in the oase of Figure 6, assume that the point a becomes first positive and then negative, due to the rectiiication within the detector 13 of the H-Af and f-Af frequencies, if the potential applied by the battery |02 to the grid 91 is such as to normally cut oil the plate current flowing through the tube, only a positive charge on the grid will cause current to now through its electromagnet |01. When the positive potential is removed at the point a the electromagnet will release. The resistor |03 connected across the input circuit of the tube 04 cooperates with the tube in such a way that when a negative charge is applied to the grid 98 its electromagnet |01 will become deenergized. Consequently, in the case of the tube 93, current through its electromagnet |01 will cause its armature |08 to pull up and make contact with its upper terminal |09, whereas in the case of tube 94 a negative charge on the grid will cause its electromagnet |01 to release, permitting its armature |53 to make contact with the lower terminal |09. The armatures |08 and the contacts |09 obviously may be connected to external circuits for control purposes. Thus the distant transmitter which generates and radiates the frequencies f-l-Af, j and f-Af can readily control the eleotromagnets |01 in any particular sequence, depending on the sequence in which the dilerent frequencies are transmitted.

In Figure S. there is shown another method and apparatus which might be used to operate a relay from the frequency sensitive detector. The elements corresponding to those described in connection with Figures 6 and 7 are identified in Figure 8 by the same reference characters. The output of the detector is taken to a polarized relay indicated at ||0. A reed is adapted to oscillate between the poles of the relay |10, this reed making and breaking the contact between the terminals H2. The rectangles ||3 represent electrical devices, including the necessary energizing voltages which are under the control of the make and break contacts at the polarized relay. The wires ||4 connect these devices with other relays and switches, depending on the type of circuit being controlled. Thus when potential at the point a is positive, asa result of the detector .13 receiving the f-l-Af frequency, the reed i may be moved upwardly to make contact. On the other hand, when the point a is at a negative potential, due to the reception by the detector 13 0f the f-Af frequency, the reed is pulled downwardly to make` contact. When the frequency f is received by the detector 13 the reed is in its neutral position because, as was explained above, the impulses of the base frequency are balanced out in the resistors 14.

In Figures 9 and 10 there are shown tWo ways of preventing fading and changes in signal amplitude from having any effect on the output Voltage from the frequency sensitive detector. However, the detector will not be greatly affected by such changes and for most purposes no special precautions will be necessary. Figure 9 shows a system for ironing out amplitude changes when necessary. A current limiting tube H5 is employed, this tube being provided with an indirectly heated cathode |I6, a grid ||1 and a plate ||8. There is a resistor ||9 connected between the grid of the plate, and the input terminals |20 are connected to any circuit which receives the f-i-Af. f and f-Af frequencies, suitable'heterodyning eiiects being provided in the receiver.

The output circuit of the limiter tube includes a transformer |2|, the primary of which is shunted by a condenser |22. There is aplate battery |23 connected between the transformer primary and the cathode H0. A variable condenser |24 is shunted across the secondary of the transformer |2| and a connection is taken to the control grid E25 of an amplifier tube |26. The latter includes an indirectly heated cathode |21 and a plate |28. The cathode is grounded through a resistor |29, and a small fixed condenser |30 is connected between the cathode |21 and the condenser |24.

The output or plate circuit of the tube |26 includes a transformer 13|, the primary of which is shunted by a variable condenser |32, and the latter is connected to the cathode through a plate battery |33 and through the resistor |29.

6 casacca A condenser |34 is connected between the negative side of the battery |33 and a grid |35 contained within the tube. There is a resistor |36 connected between the condenser |34 and the positive side of the battery. The secondary of the transformer |3| is shunted byv a variable condenser |31, and the latter in turn is shunted by a multi-pole condenser |38 which is grounded as indicated at |39. The plates '|4||`of the Vfrequency sensitive detector |4| are connected across lthe secondary of the transformer |3|,' as indicated. The detector includes a pair of indirectly heated cathodes |42, from which terminals 43 are taken; There is a wire |44 terminating in a terminal |45 and connected to the midpoint of the secondary of the transformer I3, this wire being extended through a small fixed condenser |46 to the plate |28'of the tube |26. As` in the case of the circuit shown in Figures 6, 7 and 8, the terminals |43 vmay be connected to relays which respond selectively to negative and positive direct current potentials produced in the output circuit of the detector |4|. l

Assuming that the limiting tube I5 has a fairly low plate voltage 23 and no bias due to the direct connection of the resistor ||9, the tube will quickly overload and will provide only a limited output. Once thistube has been overloaded,`no further output can be obtained from it. TheV entire circuit,` including the limiting tube," is so adjusted that Weak signals cause the limiting ftube to function, and this limited output is then amplified by the tube |25 to provide sumcient power for operating the frequency sensitive detector |4|. The direct current output vvoltage of the detector thus remains constant for a wide yrange of input voltage levels. Itis apparent that ff if `automatic voluinecontrol is also provided in the receiver, a stillwider range of inputvvoltage levels may be obtained.

In. Figure l I have shown still another circuit for limiting the current changes withinV a system employing the frequency sensitive de,tecto xj. I nstead of inserting a limiter tubebefore the ampli fying stage, as. in Figure 9, I provide aglter in the output circuit of the detector which serves to smooth out the variations of output current,

thustransmitting to the relays, which respond to therpositive andV negative polarities of the` detector ouput current, direct currents of substantially constant amplitude. In this figure thefre- Aquency sensitive detector is designated. |41, and theoutput ofthe detector is connected to an a1- tornating current lter comprising series chokes |46 and shunt condensers |49. It is apparent that Yif desired the filtery elements |48, |49 can be em- `controlling relays at a( distant receiving station.

A typical receiving station which cooperateswith the system shown in Figure 11 is illustrated in n, Figure 14. The rectangle contains any suitable and well known apparatus, such as a tube oscillator for generating the f-l-Af frequency.

The rectangle |5| designates similar apparatus ,s Y 'for generating the base or center frequency f,

|6-I, these contacts beingncrmally closed. The

right-hand side of the i`nultiswitch` bank ccmprises a spring contactmember |82 and a nxed contact member |63, these contacts being normally open; a fixed contact member |64 and a spring contact member |65, these contacts being `normally closed; and a xed'ccntact member |66'and a springY contact member |61, these contacts being normally closed. The left-hand groupv of switches is under the control of the relay |84, and the right-hand group under the control of the relay |55.

The relay |54 has a coll |68, the upper endof which is connected through a lconductor |68 through a battery |18 to the spring contact |1| of a dial switch, indicated at |16.r The xed con# tact member |13 of the dial switch is connected through a conductor |14 to the fixed contact member |66. There is a conductor |15 connected between the spring contact member |68`an`d the fixed contact |16 cfa key 4.switch indicated at |11. The spring member |18 of the key switch is connected through a conductor |18 tothe springmember |'1| of the dialswitch. i

The electromagnet Iii-is provided witha coll |80, and a conductor |8|-is taken from theupper end of the coil to theconductor |39. The conductor |82- is connected between the lower-` end of the coil andthe spring contact |5|. The lower end of the coil |58 is connected through a conductor |83 to the spring contact |51. A- wire |84 is taken from the generator |50 to-- the fixed contact |56. A conductor |85 istaken from the generator |5| to the spring contactf'li, andv a conductor |86 is connected betweenthe generator |52 and the fixed contact |63. AlY conductor |81 is connected each tothe spring contact members |51, 58 `and to the spring contact member |62, this conductor passing into a transrnittcr` |88 of any suitable and well known type. Amodulator |89 may be connected to the transmitter,

the input of the .modulator as indicated at |86,

being connected toa speech or tone modulation device. An antenna |l|- yis connected to` the transmitter. Y

In operation, when the key |92 of the dial switch is rotated through ninety` degreeathe spring member |1| makes contact with the xad contact |13. Acircuit is ,theneestahlshcd from the battery |10 through the dial switchthzmlh conductor |14, through the., contact; members a modulated carrier wave, `It fis/,assumev dtbt the carrier wave ampllfledngthe transmitter |88 is Simultaneously'being modulated by Speech or tone impulses transmitted through the'wires |90. Consequently, the antenna |9| transmits the carrier f-i-nf frequency and also the same carrier modulated by the speech or tone impulses.

At the receiving circuit, one example of which will be described in connection with Figure 14, suitable and well known devices are employed to separate the two transmitted waves. The opening of the contacts |58, |59 bythe relay |54, in effect, opens the circuit of the conductor |85 and thus prevents the base frequency f from being conducted to the transmitter |88. When the contact members |60, ISI are opened by the relay, the opposite relay |55 is rendered inoperative. It will be noted that the generator |52 cannot transmit impulses to the transmitter |38 because its circuit is opened at theA contacts |92, |53. Consequently, as the dial |92 is rotated the frequency f-l-Af, is radiated at the antenna ISI and the circuit connections are such that the base frequency f, also the differential frequency f-Af are prevented from reaching the transmitter. It is apparent that as many impulses as are necessary of the f+Af frequency may be radiated at the antenna I9! by simply rotating the dial switch.

When the key |11 is operated, contact is made at the contacts |15, |18 through the contact members |50, ll, through conductor |82, through the coil |89 and the battery |10, back to the conductor |19 to the key switch. Thus the relay |55 is energized to close the circuit between the contact members 52, |63, also to open the circuit between the Contact members |64, |55 and between the contact members |66, |61. The closing of the contacts |62, |53 closes the circuit from the generator |52 through the conductors |843, |31 to the transmitter, thus applying the f-Af frequency to the transmitter |88. The opening of the contacts |64, |65 opens the circuit through the conductor |85 of the generator |I, thus preventing the base frequency from being applied to the transmitter. The opening of the contacts |59, |61 renders the dial relay |54 inoperative. It will also be noted that the contacts |52` and |51 are open since the dial relay is deenergized; thus preventing the f-l-Af frequency from being applied to the transmitter |88. Consequently, as the key |93 is actuated the frequency f-Af is radiated from the antenna |9|.

In Figure le there is illustrated a circuit which is suitable for receiving the impulses radiated from the antenna ISIVin Figure 11. The receiving antenna is designated |04, and the carrier wave modulated by speech frequency and the f-I-Af or f-Af frequencies is passed through a standard superheterodyne receiving circuit, indicated by the rectangle |55. The speech Inodulated carrier is conducted, after the heterodyning stage, to a standard audio frequency detector and amplifier, indicated by the rectangle |96, and from there to a loud speaker (not shown) or to a modulation stage (not shown) for retransmission. Conductors |91 are taken off the main receiving circuit at a position between the heterodyning stage and audio frequency detector stage, these conductors being connected to the primary |93 of a transformer |99. The secondary 220 of the transformer is shunted by a variable condenser 20|, which is grounded as indicated at 202. The secondary 200 is connected to a grid 2l3 of a current limiting tube 204. The latter also includes an indirectly .J5

heated cathode 205, a plate 206and a grid 201. The cathode 205 is connected to the lower end of the secondary 200. There is a small xed condenser 208 connected between the cathode 205 and the grid 201.

The output circuit consists of a transformer 209,v across the secondary of which there is a variable 'condenser 2I0. A resistor 2II is connected between the condenser 2|0 and the grid 201. The secondary of the transformer 209 is shunted by a variable condenser 2| 2, and the transformer constitutes the input of an amplifier tube 2I3. The latter includes a control grid 2|4, an indirectly heated cathode 2|5, a screen grid ZIB and a plate 2|1. There is a lgrid leak 2|'8 shunted by a condenser 219 connected in the input circuit of the tube for properly biasing the control grid. A small fixed condenser 220 isiconnected between the grid leak ZIE and the screen grid 2 I 6.

The output circuit of the tube consists of the primary 22| of a transformer 222, the primary being shunted by a variable condenser 223. The secondary winding 224 of the transformer 222 is shunted by a variable condenser 225, and connections are taken from opposite sides of the condenser to the plates or anodes 225 of `a frequency sensitive detector 221.y The latter also contains indirectly heated cathodes 22S cooperating with the plates 226, as has beenexplained hereinbefore. There is a multi-plate variable condenser 229 shunted to ground and connected between the two plates 22E. The output of the detector 221 is provided with the shunting condensers 229SL and the balancing resistors 230. A wire 23| is taken from a position between the oondensers 229i, and is connected to the mid tap 232 on the secondary 224. A small fixed condenser 233 is Iconnected between the upper end of the Winding 22| and the mid tap on the secondary of the winding '224. A4 battery 234 supplies plate potential to the plate 2GB of the limiting tube and the plate 2|1 of the amplifier tube through the respective conductors 225 and 236.

The output circuit of the frequency sensitive detector 221 is taken at one side through a ground 231 and at the other side through a con ductor 23a to the grids 23s and 2in of a pair of electronic relays 24| and 242, respectively. In addition to the control grid 229, the relay 24| contains a grid 243 and a plate 2M. There is an indirectly heated cathode 261 within the tube relay, this cathode being maintained at a positive potential With respect to the ground by a battery 248. The potential and the polarity of the battery are such as to require a positive potential to be applied to the control grid 233 in order to operate the relay, i. e. to produce plate current therein. The bias potential necessary for this purpose may be carried through a resistor 245. TheY tube 242 is constructed similarly to the tube relay 24| except that the cathode 241 is maintained at approximately ground potential so that if a negative charge were applied to the control grid 240 the effect of this charge would be to reduce the plate current flowing between the plate 244 and the cathode 241. The resistor 245 serves also properly to bias the grid 24u.

The output circuit of the tube relay 241i is taken through the conductors 249 to an electromagnet 250. The output of the tube relay 242 is taken by the conductors 25| to an electromagnet 252. The movable armature 253 of the electromagnet 250 is adapted alternately to make contact with each terminal 255. The lower terminal 255 is grounded at 256. Similarly the armature 251 of the velectromagnet 252 makes contact with terminals 259, the upper one of which is grounded at 260. There is a conductor 26| connected between the armature 258 and an electromagnet 262 through a. battery 263 to ground 264.; .A conductor l265 is connected..between amature 1258 `and an electromagnet 286,'a1so through the bat.- tery 263 to ground 264. Y

There is shown a spring-biased ratchet wheel 261, the spring being of the spiral type and indicated at 268. The function of the spring'is to return the ratchet wheel 261 in a counterclockwise direction to its initial position after the latter has been moved in a clockwise direction by the action of the electromagnet, 266, as .will now be explained. Cooperating with the electromag- 'n'et 262 there is a pointer 216, ttinginto `the teeth of the ratchet wheel. A piVoted lever 21| provided with a spring-pressed pawl 212 cooperates with an electromagnet 266, the arrangement being such that as the electromagnet 266 'is energized the pawl 212 isV caused to lrotate the ratchet wheel 268 in the rclockwise direction against the returning force exercised by the spring 268. The pawl 212 .and its lever 21| are opel'.-

ated intermittently, i. e. as a step-by-step operration so that it is necessary to Vhold the ratchet wheel 261 in its last stepped position while the pawl is engaging the next tooth of the ratchet in order to turn the latter still farther.l

function is performed by the lever 268, which may be spring-biased in any suitable manner, for example, by the element 213 to hold the ratchet wheel rigidly in position until the lever 268 is Withdrawn. VThe electromagnet 282, upon energization, serves to withdraw the lever 269 and thus permit the spring 268 to restore the ratchet wheel to its initial zero position. The latter. is

i provided with an arm 214 which serves to operi and close a switch 215, depending on whether the ratchet wheel is being rotated clockwise by thepawl 212 or allowed to return in the counterclockwise direction to its initial position.

The pivot of the ratchet Wheel 261 may be constituted of a shaft on which is mounted a double-armed lever or switch 216, which is adapted to rotate as the ratchet wheel is rotated and to make contact between a number of circuits indicated at 211. These circuits may be described as telephone lines which it is desired selectively to 'connect from a distant transmitter station, as will be explained hereinafter. In case there are ten .pairs of wires at the contact bank 218, the switch 216 will connect one pair of wires to any of the ten pairs dialed.. .The shaft on which the switch levers 216 are mounted 'may be also provided with an arm which opens and closes a switch indicated at 219 as the shaft is rotated. The switch 215, which responds to the operation of the ratchet wheel 268 and the switch 219 which operates as the switch arms 216 are rotated, may be used in any suitable manner for controlling external circuits, this control being exercised in a predetermined manner depending on the transmitted signals which reachthe re-V ceiving antenna impulses generated Yat a transmitterst'ationand constituted of dierent frequencies transmitted relay |54 will have connected the generator |60 (f4-Al. frequency) .to the transmitter |88, where l theimpulses will be transmitted as a carrier and received by the antenna |94.` The carrier frequency will be heterodyned at |y (Figure 14) and the detected signal impulses will pass through the current limiting tube 204 and through the -amplier 2|3, and finally through the frequency sensitive detector 221. These impulses will be impressed on the conductor 288 as a group of positive impulses which will aEect i only thetube relay 24| and cause plate current to flow in that relay. This current will cause energization of the electromagnet 258 which draws. its armature`253 downwardly and confnects ground 256 to the conductor 265, completing the circuit through `the electromagnet 288,- the battery 263 to ground 264. Thus the electromagnet 265 is intermittently energized as many.

times as there are impulses transmitted Lthrough the frequency sensitive detector 221. The ratchet wheel 261 will be caused to rotate in the clockwise direction, due to the intermittent upward movements of the pawl 212.

It will be noted that the electromagnet 262 is deenergized all vduring this time because yits circuit is open at the upper contact 259. Consequently, the arm 268 is. forced downwardly .by

the spring 213 into the successiveratchet teeth as the ratchet wheel is rotated, in order to prevent the ratchet wheel from turning in the counter-clockwise direction due to the urge of the spring 268. As the ratchet wheel rotates five teeth in the clockwisedirection, (still assuming that ve positive impulses have been transmitted to the electromagnet 266) the switch 21lis opened and the shaft on which ythe ratchet wheel is mounted is turned through a predetermined angle to cause the switch arm` 216 of the 00ntact bank 211 to rotatev and contact with a.v predetermined pair of wires. `The rotation of the shaft may also serve to open any switch to which the shaft is mechanically connected. Consequently, the rotation of the dial |12 at the transmitting station (Figure 1l) through five steps or positions will move the switch arm 216 `of a contact bank through the same number of steps in order to select any pair of wires, kas determined by the number selected at the dial- In order to return the ratchet wheel 268 and the switch arms 216 to a zero initialiposition,

'it is simply necessary to energize the electromagnet 262, which serves .to withdraw the lever 269 from the teeth of the ratchet wheel and against the action of the spring 213. This operation may be accomplished by moving the key lss meure 11) in order to close theswitch |11.

It has been explained that when the contacts |16 and |18 are closed a frequency f-Af is applied to the transmitter y|88-and the frequency f+A-f is automatically disconnected from the transmitter. They .f-Af frequency may be transmitted as impulses of the same numberas were transmittedL by the dial |12,'or if desired as one relatively rlong impulse. This frequency, upon being obtained from the carrier at the rectangle |95, is' transmitted through the receiving circuit and frequency sensitive detector shown 1n- Figure 14,. and appears atthe conductor 288 asa direct Assume.l f

' heterodyning stage |95 because the detector is current of negative polarity. This current has no effect on the tube relay 24| for obvious reasons, but will affect the relay 242 to reduce the normal plate current flowing through this relay. This decrease in plate current serves to deenergize the electromagnet 252, causing the lever 258 to be moved upwardly against the upper contact 259. The conductor 26| is then grounded,

'completing a circuit through the electromagnet 262 and the battery 263 to ground. Energize,- tion of the electromagnet 262 will Withdraw the lever 29 from the ratchet wheel either as a step-by-step operation, depending on the character of the impulses produced at the key |93 (Figure 11), or completely remove lever 269 from the ratchet Wheel to permit the latter to return immediately to its zero or initial position ready for the next dialing operation.

As in the case of the figures previously discussed, the frequency sensitive detector 221 serves d to rectify either the f-i-Af or ,f-Af frequencies and to convert these frequencies into direct current potentials, either of positive or negative polarity depending on the particular frequency. The tube relays 24| and 242 discriminate between these impulses of opposite polarity selectively to operate 1 apparatus, namely, the ratchet wheel 268 land the contact bank 218, either in the clockwise or counter-clockwise direction depending on the frequency received at the detector 221. o

It is also apparent that the switching functions performed by the detector 221, as explainedabove, are not aiected by the presence of the voice modulated carrier which passes through the sensitive to changes in frequency only, and not to changes in amplitude. The only effect that changes in amplitude can have will be to cause slight changes in the amplitude of the output voltages from the frequency sensitive detector, but these changes do not change the polarity. Even these slight changes in amplitude of the voltages may be removed, if desired, by small filter condensers placed across the output circuit of the detector, as explained in connection with Figure 10, or a limiter tube for maintaining constant voltage to the input of the frequency sensitive detector may be employed as indicated at 204 (Figure 14). g

The devices and systems described hereinbefore are almost entirely free from static interference which normally produces false operation of the remote relays. The detector tube 221 is sensitive to changes in frequency only, whereas static is composed of energy having large proportional changes of amplitude. Any increase in voltage that is produced by static at the output of the detector tube 221 (Figure 14) is of a very transient character and of short duration, and may be still further reduced, if desired, by using filters to delay the operating speed of the switching mechanism.

It will be understood that I desire to comprehend within my invention such modifications as come within the scope of the claims and the invention.

Having thus fully described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In a composite receiving system, means for receiving a carrier of the frequencies f-i-Af, f and f-Af which is modulated by voice undulations, means for separating the voice undulations from said carrier and for transmitting the voice undulations to a voice-frequency-responsive device, a separate means for transmitting said carrier frequency to a device which responds to difference of frequency, said last-mentioned device comprising a double-unit rectifier whichk serves to convert the frequencies ,f-l-Af and f-Af into direct current impulses of opposite polarities, said reotier also serving to eliminate the frequency, and means responsive to said direct current impulses of opposite polarity for controlling the device which responds to said voice undulations.

2. In combination, a transmitting station comprising a source of frequencies f, f-i-Af and f-Af and an outgoing line, dial means for transmitting over said line a succession of impulses of the -l-Af frequency, said means also serving to prevent the transmission of the ,f frequency, key means for applying to said line an impulse of the JAf frequency at the conclusion of the 4fel-'Af frequency impulses, said key means also serving to prevent the transmission of the f frequency.

3. In combination,- means for receiving frequencies f-l-Af and -Af from a transmitting circuit, a dial means for transmitting said frequency f-l-Af, and a keying means for transmitting said frequency f-AI', said receiving means including a relay which responds cumulatively to the dialtransmitted succession of impulses and becomes deenergized when the key-transniitted impulse is impressed thereon.

4. In combination, means for receiving a plurality of frequencies f-l-Af and f-Af, dial means for transmitting the f-l-Af frequency to the receiving means in the form of a succession of impulses, keying means for transmitting said f-Af frequency to the receiving nieans at the conclusion of the j-l-A frequency, said receiving means comprising a relay Which responds cumulatively to the f-l-Af impulse and becomes deenergized when the f-Af impulse is impressed thereon.

5. In combination, a receiving circuit adapted to receive a plurality of signaling frequencies f, f-l-Af and f-Af modulated by a voice frequency responsive device, means for separating the voice undulations from said signaling frequencies, and means for conducting the carrier frequencies through an amplifier to a double-unit rectifier which converts the signal frequencies into direct current impulses of opposite polarity, a relay responsive to the received frequency H-Af, a different relay responsive to the received frequency f-Af, and a device for receiving said signal frequencies, said device being adapted to be actuated' in one direction upon the receipt of the f|Af frequency and to be Iactuated in the opposite direction upon the receipt of the f-Af frequency, said device serving to control an outgoing circuit in accordance with the direction in which it is actuated, said outgoing circuit being constituted at least in part of mechanism by which the voicefrequency-responsive device is controlled.

ROBERT A. 

